Capillary-level perfusion is a key physiological parameter and its measurement is vital to the complete understanding of organ function. The usefulness of perfusion measurements is readily demonstrated by the wide range of methods currently used in research labs and medical practice. Almost all of these methods rely on the use of exogenous contrast agents, which suffer from a number of disadvantages that complicate interpretation and limit utility. We have developed a unique and completely non-invasive NMR perfusion measurement technique, which we call FAWSETS (Flow-driven Arterial Water Stimulation with Elimination of Tissue Signal) FAWSETS is sensitive enough to provide the temporal and spatial resolution required for clinical applications and specific to perfusion so that quantification is feasible with a simple model. The goal of this project is to further develop and optimize FAWSETS and demonstrate its value as a clinical as well as research tool. Specific aim #1 will focus on enhancement of perfusion sensitivity. This will require construction of custom RF/gradient coils and extensive testing in flow phantoms. In specific aim #2 we will conduct measurements in skeletal muscle that will define the specificity of the measurements in humans. Skeletal muscle is unique among organs in that prolonged ischemia can be safely and easily induced. This permits simple but vital tests for specificity that cannot be conducted in any other organ. Specific aim #3 will focus on sensitivity to perfusion. Our brain imaging experiments will demonstrate sufficient sensitivity for an important clinical application: the ability to monitor cerebrovascular reserve during the acetazolamide challenge. This project consists of a carefully considered plan to demonstrate the value of our method as a tool with potential for broad application. We are confident that our methods will later be adapted to test physiological hypotheses in a variety of organs where unambiguous information on parenchymal perfusion is needed. Our specific aims focus on two such organs. We have chosen to develop the measurements in skeletal muscle and brain because of their particular circulatory properties and because both organs are studied routinely and extensively in this research group. Our intention is to expand this work into significant clinical and basic science programs in future grants.